The present invention relates to security documents such as bank notes or the like with security indicia for preventing unauthorised reproduction of the security document, and is particularly concerned with security documents having security indicia which are detectable in transmitted light but are less apparent in reflected light.
A wide variety of security devices or features for security documents, such as bank notes, traveller""s cheques or the like have been proposed previously. Such security devices are provided in order to make falsification and counterfeiting of the security documents difficult.
Amongst those considered most effective for use in bank notes and like security documents are security devices which present authentication or security indicia in transmitted light but which act to make such authentication indicia less visible in reflected light. Many conventional security devices of this type, however, have the disadvantage of being difficult or complicated to produce or relatively easy to counterfeit.
U.S. Pat. No. 5,161,829, for example, describes a security paper including two layers of paper each having inner and outer surfaces and being of predetermined mechanical properties, colour and optical density. Authentication indicia are printed on the inner surface of one of the layers, and an adhesive permanently adheres the inner surfaces of the two layers together. The optical properties of both layers, of the adhesive and of the medium constituting the indicia are selected to render the security indicia detectable in transmitted light and substantially imperceptible in reflected light. Such an arrangement, however, is complex, costly to produce and restrictive in the choice of materials and colours which may be used in the production of bank notes to which such a security device is applied. None of the known security documents of the above mentioned type include a security device which is detectable in transmitted light and less evident in reflected light which is simple and convenient to manufacture and which does not unnecessarily limit or dictate the choice of materials used in the security document in which it is incorporated.
Accordingly, the present invention provides a security document with security indicia for preventing unauthorized reproduction, the security indicia being formed of at least partly transparent windows formed through the security documents and being detectable in transmitted light, wherein the security indicia are located within the bounds of a security pattern acting to visually conceal the security pattern in reflected light.
Preferably, the security pattern is formed of one or more elements and has a complexity selected to enable the concealment of the security indicia.
Various characteristics of the security pattern may be selected in order that an appropriate level of complexity to conceal the security indicia is achieved In one embodiment of the invention, the number of elementier unit area, or density of the elements, is selected to enable the concealment of the security indicia.
In addition, the irregularity of the security elements may be selected to enable such concealment
One or more of the dimensions of the elements may be selected to enable such concealment.
Preferably, the security pattern is formed on a background surface by means of gravure, offset or intaglio printing techniques.
In at least one embodiment of the invention, the security pattern is formed on a background surface, the colours of the security pattern and the background being selected to contribute to the concealment of the security indicia Preferably, colours which are highly contrasting, and which thus act to confuse the eye of the viewer, will be chosen.
It is preferable that the security indicia themselves will also have a complexity selected to contribute to the concealment of the security indicia Such security indicia, for example, should preferably not be comprised of solid shapes such as circles or squares but should be more complex in form.
Preferably, the security indicia have a width and/or height in the plane of the security document of less than 2.5 mm.
Ideally, the security indicia and the security pattern will have similar complexities in order to optimise the concealment of the security indicia
In at least one embodiment, the security document has a surface to which printed matter is applied, and an intaglio print is formed on the surface, the security indicia being located within the bounds of the intaglio print. The intaglio print, because of the height of the intaglio lines, acts to conceal the security indicia at oblique angles in reflected light.
In a preferred embodiment of the invention, the security document comprises an at least partially transparent substrate having first and second opposing faces, and one or more opaque layers applied to at least one of the faces. The windows may be comprised of apertures formed through one or more of the opaque layers.
The following description refers in more detail to the various features of the present invention. To facilitate an understanding of the invention, reference is made in the description to the accompanying drawings where the security document is illustrated in a preferred embodiment. It is to be understood that the security document of the present invention is not limited to the preferred embodiment as illustrated in the drawings.